A one-piece golf ball or a core for a multi-piece golf ball whose a representative example is a two-piece golf ball is generally obtained by introducing a rubber, a metal salt of .alpha.,.beta.-unsaturated carboxylic acid as a co-crosslinking agent, a filler, an organic peroxide, and the like into a kneader or a Banbury mixer, and mixing them to obtain a rubber composition, followed by vulcanizing or press-molding. However, since the all components for the rubber composition are simultaneously poured into the mixer, mixing is difficult and dispersibility is poor. Since the mixing temperature is as high as more than 130.degree. C., the decomposition of the peroxide and the reaction of the metal salt of .alpha.,.beta.-unsaturated carboxylic acid with the metal oxide as a filler would occur. When the reaction occurs, the rebound characteristics of the golf ball obtained by molding the resulting rubber composition is degraded.
In order to solve the problem, it is attempted that the co-crosslinking agent and the filler (metal oxide) are separately mixed with the base rubber and the like and then the both are blended together with immediately before extruding, as described in Japanese Patent Kokai Publication 319373/1992, but the workability is degraded because of two separate mixing processes.